The present invention relates to an image printing apparatus using a plurality of laser beams (multi-laser beam), and more particularly, the invention relates to art edge smoothing circuit for use in an image printing apparatus.
Image printing apparatuses using a laser beam can print with a higher speed and a higher resolution compared to printing apparatuses of other types, and, accordingly, are widely used. In the past, in a image printing apparatus of this type, a semiconductor laser generating a laser beam having a wavelength of 780 nm has been used, and the printing beam spot diameter becomes 50 to 100 .mu.m when using a general scanning optical system, and, consequently, the resolution of the printing apparatus becomes 300 to 600 dpi. However, as discussed in a paper presented in Applied Physics of Japan, Vol. 16, No. 7 (1996) pp 676-785, the printing beam spot diameter can be decreased to nearly one-half of the conventional printing beam spot diameter by using a GaN semiconductor laser which generates a laser beam having about one-half of the wavelength (420 nm) of the conventional semiconductor laser. Therefore, an image printing apparatus having a higher resolution can be obtained when using such a laser.
However, in this case, the printing speed of the image printing apparatus is decreased, because the scanning beam density has to be increased at the same time. For example, when an image printing apparatus having a scanning beam density of 300 dpi is changed so as to have a scanning beam density of 600 dpi, the printing speed becomes one-half. Although the printing speed can be increased by doubling the rotating angular speed of the rotating polygon mirror for deflecting the laser beam in the image printing apparatus, it is difficult to further increase the rotating speed because the rotating speed is already at the upper limit of the speed range in the high speed printing apparatus. Therefore, Japanese Patent Application Laid-Open No. 8-15623, has proposed an image printing apparatus wherein high speed or high resolution can be attained by increasing the number of laser beams performing the scanning exposure.
On the other hand, in regard to an image printing apparatus using one laser beam, an image quality improving method has been proposed in Japanese Patent Application Laid-Open No. 8-310057, which calls for smoothing a slanting portion in an outline of a character or an image so as not to generate jags by making use of the characteristics that an image printing apparatus using a laser beam can modulate laser intensity continuously in the main scanning direction and can increase or decrease the toner attaching quantity depending on the laser intensity.
FIG. 2 is a block diagram showing a typical system in which a common image printing apparatus is used. A user forms page description data 202 identifying contents of pages to be printed using a data forming apparatus 201, such as a computer. On starting printing, the page data 202 is transmitted to a controller 203 of the image printing apparatus through a network or the like. The controller 203 expands the page description data 202 for every page on a bit map memory as image data 204. Therein, it is assumed that the image printing apparatus is a monochromatic binary laser printer, and the image data 204 is binary data corresponding to 1-pixel per 1-bit. When the expansion of the image data 204 is completed, the controller 203 starts an engine 205 of the image printing apparatus, and then transmits the image data 204 to the engine 205 in response to a synchronous signal 206 received from the engine 205. The engine 205 prints an actual image on a print medium according to the image data 204.
FIG. 3 is a block diagram of an image printing apparatus using one laser beam. This image printing apparatus has an edge smoothing circuit 301 connected between the controller 203 and the engine 205, as disclosed in Japanese Patent Application Laid-Open No. 8-310057. The edge smoothing circuit 301 may be installed inside the controller 203 or inside the engine 205. Therein, among the synchronous signals 206 transmitted from the engine 205 to the controller 203, a signal for synchronizing a scanning position of the laser beam on the scanning line with the image data 204 is referred to as a line synchronous signal BD. Each of a plurality of laser beam detectors is arranged at a position just before a position starting each line of printing. The line synchronous signal BD is a signal generated when the laser beam being deflected and scanned passes through the laser beam detector. The edge smoothing circuit 301 receives the line synchronous signal BD, a pixel clock signal DCLK in synchronism with the image data 204, a high frequency pixel clock signal HCLK and an image data signal VD, and outputs a laser modulating image data signal enhanced for edge smoothing.
FIG. 4 shows a timing chart for each of the signals iin the controller 203. Signal forming with such timing can b,e performed by inputting a clock signal CLK of 50 MHz into an IC (M66235FP: a product of Mitsubishi Electric Corp.) when the printing speed of the image data 204 in the engine 205 is assumed to be 25 M pixels/second (reference: '95 Mitsubishi Semiconductor Data Book, edited by Digital ASSP).
The clock signal CLK of 50 MHz generated by an external quartz oscillator is continuously input to the IC (M66235PF). The line synchronous signal BD is input from the engine 205 out of synchronism with the clock CLK. The IC (M66235PF) generates a clock signal HCLK by synchronizing the phase of the clock CLK with the line synchronous signal BD with a delay represented by a time period of .DELTA.T and a clock signal DCLK having one-half frequency of the clock signal HCLK. Accuracy of the synchronization is .+-.3 nsec, which is sufficient when taking it into consideration the fact that the frequency of the pixel clock signal DCLK is 25 MHz. The image signal VD is transmitted to the edge smoothing circuit 301 in synchronism with the pixel clock signal DCLK with 1-pixel per 1-bit signal d0 (d2, . . . ).
FIG. 5 shows the construction of an edge smoothing circuit 301 in the conventional apparatus (Japanese Patent Application Laid-Open No. 8-310057). The edge smoothing circuit 301 can be roughly divided into a line memory 501, a logic circuit 502 and a pulse width modulation circuit (hereinafter, referred to as PWM) 503. Although the detailed operation is to be described later, an outline thereof is as follows.
The line memory 501 temporarily stores image data signals VD for a plurality of scanning lines, and transfers image data signals VD of pixels to be printed (reference pixels) and several pixels around the reference pixels to the logic circuit 502 as a single unit. The logic circuit 502 judges by template matching what edge the reference pixels compose a part of, and the judged result is transferred to the PWM 503. The PWM 503 generates enhanced image data (laser modulation) signals Vde for printing the reference pixels by adding appropriate pulse width modulation based on the judged result.
FIG. 6 is a block diagram showing the construction of an image printing apparatus using a plurality of laser beams. This image printing apparatus is composed of a controller 601 and a printer engine 602. A detailed description will be omitted here, since the details are disclosed in Japanese Patent Application Laid-Open No.815623. The controller 601 supplies the engine 602 with image data signals VD1 to VD4 in synchronism with line synchronous signals BD respectively corresponding to plural laser beams.
FIG. 7 is a perspective view showing the construction of an optical system in an engine 701 of an image printing apparatus of the electrophotographic printing type. Here, it is assumed that the image printing apparatus uses four laser beams 701. The four laser beams 701 must be incident onto a rotating polygon mirror 702 by providing four lasers, as disclosed in Japanese Patent Application laid-Open No.6149346, or by splitting one laser beam into four laser beams. As shown in the figure, the four laser beams 701 are focused onto the surface of a photosensitive drum 703 to form beam spots, and the beam spots are deflected and scanned in a main scanning direction (an axial direction of the photosensitive drum). Since there are four laser beams 701, four scanning lines 704 can be printed during one scanning. Since the rotating drum 703 is rotating, the direction transverse to the rotating direction of the photosensitive drum 703 is the sub-scanning direction.
On the photosensitive drum 703, if we let the laser beams 701 from the upstream side of the sub-scanning direction be laser beam (1) to laser beam (4), respectively, and let scanning positions of the respective laser beams be scanning position laser beam (1) 706 to scanning position laser beam (4) 709, then four line synchronous signals BD are generated by one scanning since each of the laser beais (1) to (4) crosses over a beam detector 705 in the top front position during the one scanning. The first one is a line synchronous signal BD1 produced by the laser beam (1), and then follows a line synchronous signal BD2 produced by thus laser beam (2), a line synchronous signal BD3 produced by the laser beam (3), and a line synchronous signal BD4 produced by the laser beam (4).
In FIG. 6, the engine 602 transmits a compound line synchronous signal BD of the line synchronous signals BD1 to BD4 to the controller 601. The controller 601 splits the received line synchronous signal BD into the four line synchronous signals BD1 to BD4. The method of splitting the compound signal is disclosed in Japanese Patent Application Laid-Open No. 8-15623. The controller 601 transmits image data signals VD1 to VD4 respectively synchronized with the line synchronous signals to the engine 602 as printing (laser modulating) signals respectively corresponding to the laser beams (1) to (4).